I'd say Harry Potter was one that really pushed my imagination.
However, less popular books I'd recommend are on a typical middle school's reading list.
Walk two moons - teaches a lot about think about others
A mid summer night's dream - basic Shakespeare to introduce children to literature
Roll of thunder hear my cry - fantastic book about social prejudices
To kill a mockingbird - just one of the best books ever written. this is considered a classic.
There are many more. Just search for Middle School Reading list for more.